Fix a complaint from clippy

[psychon]

error: the borrowed expression implements the required traits
  --> x11rb/tests/resource_manager.rs:96:13
   |
96 |             &dir,
   |             ^^^^ help: change this to: `dir`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
   = note: `-D clippy::needless-borrow` implied by `-D warnings`

---

No idea why beta clippy only cares about this one instance. This file is full of more cases of the same, I think. But whatever.

New Rust release, new PR to please the clippy god.

Edit: Ah, I know why. Otherwise one gets "use of moved value"-errors.

[codecov]

Codecov Report

Base: 13.54% // Head: 13.54% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (1e5dff3) compared to base (7d0dafa).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #762      +/-   ##
==========================================
- Coverage   13.54%   13.54%   -0.01%     
==========================================
  Files         141      141              
  Lines      119908   119908              
==========================================
- Hits        16244    16243       -1     
- Misses     103664   103665       +1     

Flag	Coverage Δ
tests	13.54% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
x11rb/tests/resource_manager.rs	63.67% <100.00%> (ø)
x11rb/tests/multithread_test.rs	90.60% <0.00%> (-0.68%)	⬇️

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